The PMMA surface by itself is not too reactive. You have to activate it e.g. by air plasma treatment, resulting in surface -OH groups that MAY react with silanes but you will have optimize the reaction conditions. You may also try to add tetramethyl silane to argon plasma in low concentration - although I am not sure how strong the plasma-polymerized layer adhere to the surface. But the best solution is to copolymerize 3-(Trimethoxysilyl)propyl acrylate wih methyl methacrylate. It will concentrate on the surface.

The surface of PMMA can be treated by radio-frequency (RF) discharge plasma in argon and subsequently grafted by vinylalkoxy silane to form a surface containing new polar functional groups. The decrease of surface energy of PMMA modified by plasma in the course of aging should be stabilized by silane grafting. The modification of PMMA in argon RF plasma with tetraethyl (methyl) silane is also suitable to use.